The present invention relates to molded carrier rings for electronic packages. More particularly, the invention relates to a two piece plastic ring which surrounds the external leads of a leadframe preventing distortion.
Electronic packages for housing integrated circuit devices such as silicon based semiconductor circuits, called chips, are increasingly required to have a high density of electrical interconnections. One high density package is known as the quad flat pack. The package is square or rectangular with a plastic, ceramic or metal body having leads approaching from four sides. The quad package has a metal leadframe with a central paddle for chip attachment. In one method of assembly, the chip is attached using a silver filled epoxy or solder. The chip is then electrically connected to the leadframe by wire bonding or tape automated bonding. From about 10 to 100 leads extend from each side of the central pad resulting in package lead counts of 40 to 400 leads or more. The centrally positioned paddle, chip and inner leads are then encapsulated in an epoxy molding resing or between metallic base and cover components with outer lead ends extending from the package body. Transfer molding is usually used. The low viscosity of the molding resins minimizes damage to bond wires.
The leads are manufactured from thin gauge copper, a copper alloy or an iron-nickel alloy. With a thickness of from 0.001 to 0.010 inches and typically about 0.005 to 0.006 inches, the leads are much more fragile than the leads of a conventional dual-in-line (DIP) package which has leads with a thickness of 0.010 inches. The leads are susceptible to damage during device testing and mounting.
To minimize lead distortion during shipping, handling and testing, a carrier ring is molded around the external ends of the leads. As disclosed in U.S. Pat. No. 4,701,781 to Sankhagowit, a molded carrier ring and package body are simultaneously encapsulated by transfer molding of an epoxy resin. The ring remains in place during testing and shipping preventing distortion of the leads.
The molded carrier ring is effective, but suffers from several limitations. The outer leads are encapsulated at the same time as the chip, paddle, and inner leads. This mandates that the molded carrier ring be made from the same molding resin as the semiconductor package body. To change encapsulation resin requires a two step process adding to the tooling and equipment costs, increasing processing costs and reducing the yield.
To prevent non-uniform flow of resin in both the package body and carrier ring, dam bars are required between each lead at the outer edge of the package body and also at the inner edge of the molded carrier ring. Both sets of dam bars must be trimmed to provide electrical isolation between leads. The molded carrier ring doubles the number of dam bars which must be removed. The process for removing dam bars involves the use of highly detailed punch tools which are expensive to build and maintain.
The epoxy resins used to encapsulate semiconductor devices shrink as the resin polymerizes, typically by about 0.4%. If the quad flat pack or the ring is not symmetric with equal resin thickness above and below the frame, shrinkage may warp the leadframe. Resin shrinkage also distorts lead-to-lead spacing, or pitch. The variation in pitch is a problem for dam bar removal, electrical testing and board mounting. The variations in pitch become a more significant problem with higher lead count and finer pitch leadframes.
In a metal quad flat pack, such as disclosed in U.S. Pat. No. 4,939,316 to Mahulikar et al, the quad leadframe is disposed between metallic base and cover components and bonded to both with an epoxy adhesive. Dam bars are not required. The use of a carrier ring molded about the leadframe requires the addition of dam bars to the leadframe. The addition of dam bars adds the process step of severing after encapsulation, increasing the number of processing steps and adding to tooling costs.